Characteristics and Dynamics of wind-driven upwelling. in the Alaskan Beaufort Sea based on six years of mooring data

Transcription

1 1 2 Characteristics and Dynamics of wind-driven upwelling in the Alaskan Beaufort Sea based on six years of mooring data 3 4 Peigen Lin 1,2, Robert S. Pickart 2, G.W.K. Moore 3, Michael A. Spall 2, Jianyu Hu November 217 Revised for Deep-Sea Research II Corresponding Author: Peigen Lin (plinwhoi@gmail.com) State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, Fujian 36112, China 2 Woods Hole Oceanographic Institution, Woods Hole, MA 254, USA 3 Department of Physics, University of Toronto, Toronto, Ontario, Canada. 23

2 Abstract Six years of mooring data from the Alaskan Beaufort Sea slope, together with meteorological observations and reanalysis fields, are used to quantify the occurrence of wind-driven upwelling and the associated atmospheric forcing. The canonical upwelling event, composited from 115 individual events, reveals that when the easterly wind is strongest the entire shelfbreak jet is reversed to the west. At the end of the event a bottom-intensified, eastward-flowing rebound jet spins up that is stronger than the normal shelfbreak jet. The cross-isobath flow has a three-layer structure with onshore flow in the surface layer, offshore flow in the middle of the water column, and onshore flow near the bottom. This is because the reversed shelfbreak jet is oriented slightly onshore which overwhelms the cross-isobath surface Ekman transport. The vertically-integrated along-isobath momentum balance supports this interpretation and also indicates that the rebound jet is driven by the zonal gradient in sea surface height. During over two thirds of the events Atlantic water (AW) is upwelled to the shelfbreak, while for the remaining events only Pacific water (PW) is upwelled. The primary driving factor behind this is the seasonal variation in the PW-AW interface depth offshore of the shelfbreak, which is controlled by the local wind stress curl. During summer, when PW-type events dominate, Ekman pumping associated with negative wind stress curl deepens the interface depth, limiting access to the Atlantic layer. Over the remainder of the year, when AW events dominate, Ekman suction associated with positive wind stress curl raises the interface. These variations are due to the influence of the two regional atmospheric centers of action the Aleutian Low and the Beaufort High Keywords: upwelling, Beaufort Shelfbreak jet, shelf-basin exchange, atmospheric centers of action

3 Introduction It has long been known that upwelling occurs in the Alaskan Beaufort Sea, driven by easterly winds (Hufford, 1974). The upwelling takes place throughout the year during a variety of ice conditions. Using two years of mooring data together with meteorological records, Schulze and Pickart (212) demonstrated that roughly 95% of strong storms (where easterly wind speeds exceed 1 m s -1 ) result in upwelling. Based on this empirical relationship, Pickart et al. (213a) used the 7-year wind record from the Barrow, AK weather station to characterize long-term trends in such strong upwelling storms. According to this proxy, in recent decades upwelling events have increased in both number and strength. In addition, Pickart et al. (213a) determined that there are two seasonal peaks of upwelling occurrence associated with enhanced easterly winds, one in spring (May) and a second in fall (November). However, these two peaks have shifted in recent years: the spring peak is now later (June), and the fall peak earlier (October), associated with a corresponding shift in alongcoast windspeed (Lin et al., 216). Wind conditions in the Alaskan Beaufort Sea are primarily dictated by the behavior of two atmospheric centers of action the Aleutian Low and the Beaufort High (Pickart et al., 213a; Brugler et al., 214). The latter is a semi-stationary region of high pressure situated in the central Beaufort Sea (Reed and Kunkel, 196; Walsh, 1978), which gives rise to easterly winds in the southern Beaufort Sea. Lin et al. (216) demonstrated that the Beaufort High is the main driver of upwelling during the warm months of the year, consistent with the results of Watanabe (213) and Pickart et al. (213a). The Aleutian Low is the integrated signature of low pressure systems transiting along the north Pacific storm track (Wilson and Overland, 1986; Zhang et al., 24). During the passage of such storms, easterly winds in the southern Beaufort Sea are often enhanced. The Aleutian Low becomes deeper in the fall and winter (Favorite et al., 1976). While this

4 contributes to the autumn wind peak noted above, both centers of action play a role during the cold months of the year (Pickart et al., 213b; Lin et al., 216). While upwelling can occur in any month of the year, it is strongly modulated by the presence of sea ice (Schulze and Pickart, 212). Since mobile pack ice is able to transmit wind stress to the water column (Røed and O'Brien, 1983), upwelling readily occurs when the sea surface is only partially covered by ice. In fact, Schulze and Pickart (212) found that upwelling was strongest during the shoulder ice seasons (late-spring and late-fall). This is in line with the numerical results of Martin et al. (214) and the observations of Pickart et al. (213b). The reason for this is that the momentum flux from the atmosphere to the ocean is stronger via freely-moving ice keels (Häkkinen, 1986; Pite et al., 1995). Notably, upwelling at the Beaufort shelfbreak occurs even when the ice concentration is 1%, as long as the pack ice is mobile (Schulze and Pickart, 212). Wind-driven upwelling is one of the primary mechanisms of shelf-basin exchange in the Alaskan Beaufort Sea. The resulting cross-stream circulation transfers heat and freshwater offshore into Canada Basin (Yang, 26). According to Pickart et al. (213b), a single (strong) storm can flux enough heat offshore to melt an area of 1-m thick ice the size of the Beaufort shelf. At the same time, the freshwater fluxed offshore is enough to significantly influence the year-toyear variations in the freshwater content of the Beaufort Gyre (Proshutinsky et al., 22, 29). In addition to these physical effects, the wind-driven exchange has ramifications for the regional ecosystem as well. Nutrients are upwelled from the basin (Pickart et al., 213b) that could impact primary production on the Beaufort shelf (Macdonald et al., 21). This in turn provides food for Arctic cetaceans (e.g. Okkonen et al., 211). The boundary current system of the Beaufort Sea consists of two components. The first is the Beaufort shelfbreak jet, which is variously referred to in the literature as the western Arctic

5 boundary current (Nikolopoulos et al., 29), or the Pacific-water boundary current. As reported in previous studies (e.g. von Appen and Pickart, 212; Brugler et al., 214), the shelfbreak jet advects both Pacific summer water and winter water, the latter of which contains high concentrations of nutrients (Lowry et al., 215). In late-summer the current is surface-intensified and transports very warm and fresh Alaskan coastal water. The rest of the year it is bottomintensified and advects Bering Sea summer water (in early-summer and again in early-autumn) and winter water (the remainder of the year). The eastward volume flux, heat flux, and freshwater flux are all greatest in summer when the current is surface-intensified (Brugler et al., 214). This is consistent with studies on the Chukchi shelf indicating that, in summer, much of the Pacific water emanating from the Bering Strait exits the shelf through Barrow Canyon (Itoh et al., 212; Gong and Pickart, 216; Pickart et al., 216, Weingartner et al., 217). The second component of the Beaufort Sea boundary current system is the eastward-flowing Atlantic water located downslope and offshore of the shelfbreak jet. On average, the Atlantic water resides deeper than 18 m beneath the layer of Pacific winter water (Nikolopoulos et al., 29). The Atlantic water is a part of the large-scale cyclonic boundary current system of the Arctic Ocean (Aagaard, 1984; Rudels et al., 24; Karcher et al., 27; Aksenov et al., 211). Presently its eastward transport in the Canada Basin is unknown. This is because, unlike the shelfbreak jet, it has not been adequately sampled by moorings. Only the inshore portion of the current has been documented observationally (Nikolopoulos et al., 29). During easterly wind events the shelfbreak jet typically reverses to the west, followed a short time later by upwelling of water from the basin to the shelf (Pickart et al., 29, 213b). In some cases the upwelled water is Pacific water, while at other times warm and salty Atlantic water is also upwelled (Schulze and Pickart, 212). To date, the reasons dictating one scenario versus the

6 other have not been explored. Using data from a cross-stream array of 7 moorings spanning the outer Beaufort shelf to the mid-continental slope, Pickart et al. (211, 213b) diagnosed a single event that occurred in November 212. This revealed that, while the isopycnals in the upper part of the water column were deflected upward onto the shelf during the event, the deeper isopycnals were deflected downwards. This highlights the fact that the upwelling is not simply a twodimensional process. Furthermore, during the relaxation phase of the November 212 event, a deep eastward-flowing jet was spun up, temporarily enhancing the flow of Atlantic water. It remains to be determined if both of these features are ubiquitous to upwelling on the Beaufort slope. In this paper we use 6 years of mooring data to quantify various aspects of upwelling across the Beaufort shelfbreak. The first two years of data (22 24) come from the mooring array noted above, while the remaining four years (28 212) are from a single mooring deployed in the center of the Beaufort shelfbreak jet. With this amount of data, we can, for the first time, construct and diagnose a composite upwelling event and quantify the hydrographic response as well as the primary and secondary circulation. Two of the major aims of our study are to understand the cross-isobath flow during an upwelling event, and determine the underlying reason why some of the events advect only Pacific water onto the shelf, while others flux Atlantic water as well. In doing so we elucidate the role of the atmospheric forcing, both on synoptic timescales as well as seasonally. The location of the mooring array (and the single mooring in the later years) corresponds to one of the designated sections of the Distributed Biological Observatory (DBO) program. As described by Moore and Grebmeier (217), DBO is designed to detect and ultimately understand ongoing changes to the ecosystem of the Pacific Arctic. Towards this end, eight locations have

7 been identified, spanning the region from the northern Bering Sea to the Canadian Beaufort Sea, where targeted measurements are being carried out by the international community. This includes shipboard sampling via dedicated programs or cruises of opportunity, as well as mooring measurements. The specific locations were chosen in part due to enhanced biological activity in the water column and/or the sediments. The mooring data described in the present study were collected at the DBO-6 line, which crosses the Alaskan Beaufort shelf/slope approximately 15 km east of Barrow Canyon (see Moore and Grebmeier (217) for the locations of the eight DBO lines). This region is characterized by significant shelf/basin exchange due to both external forcing (wind) and internal forcing (instabilities of the flow). As such, the present study serves to inform the DBO community regarding various aspects of the wind-driven exchange at this location and its potential impacts on the ecosystem Data and methods 2.1. Mooring array data in A mooring array was maintained across the Alaskan Beaufort shelfbreak and slope (near 152 W) from August 22 to early September 24 as part of the Western Arctic Shelf-Basin Interactions (SBI) program (Fig.1). There were 8 moorings named BS1 BS8 from onshore to offshore (BS6 was only available from August 22 to early September 23). In this study we only use moorings BS2 BS7. The velocity was recorded hourly using upward-facing acoustic Doppler current profilers (ADCP, RDI 75 KHz) with a vertical resolution of 5 1 m at moorings BS2 BS6. Hydrographic traces were obtained 2 4 times per day using a coastal moored profiler at these sites and a McLane moored profiler at BS7. These profilers are motorized conductivitytemperature-depth (CTD) instruments that provide vertical profiles with 2 m resolution spanning

8 from the top float of the mooring (at roughly 4 m depth) to near the bottom. Velocity at mooring BS7 was measured using an acoustic travel-time current meter that was part of the profiler. Hourly point CTD measurements were collected at all of the sites using MicroCats, which were situated just below the deepest profiler depth. The reader should consult Fratantoni et al. (26), Spall et al. (28), and Nikolopoulos et al. (29) for details of the instrumentation on the moorings, including the calibration procedures and measurement accuracies Single mooring data in One of the moorings in the array, BS3 (blue star in Fig. 1), which is in the center of the Beaufort shelfbreak jet, has been subsequently deployed since August 28 as part of the Arctic Observing Network (AON). The deployments were similar to that in the years As seen in Table 1, the top float was situated between 35 4 m, and the upward-facing RDI 75 KHz ADCP and MicroCat were located at 128 m depth (roughly 2 m above the bottom). The vertical range of the coastal moored profiler had small changes from year to year depending on the top float depth. Starting in 29 a second upward-facing ADCP (RDI 3 khz) was attached to the top float along with an additional MicroCat. The vertical resolution of the shallow velocity profile was 5 1 m. We interpolated the velocity data using both ADCPs in when the top instrument returned good data, otherwise only data from the 75 khz instrument were used (whose range extended close to the surface) Meteorological data Wind data used in this study are from the meteorological station in Barrow, AK (Fig. 1), which is the closest weather station to the mooring array. Previous studies (e.g. Nikolopoulos et al., 29; Pickart et al., 29) have demonstrated that the winds at Barrow are a good proxy for those at the array site. The data were obtained from the National Climate Data Center of the

9 National Oceanic and Atmospheric Administration ( and have been quality controlled and interpolated to a one-hour temporal resolution (see Pickart et al., 213a) Atmospheric reanalysis data We use wind stress and sea-level pressure (SLP) data from the Arctic System Reanalysis (ASR) version 1, which has a 3 km grid spacing and 3-hourly temporal resolution, available at NCAR Research Data Archive ( The ASR is based on the Polar WRF model with boundary conditions provided by the Interim Reanalysis from the ECMWF (ERA-I) (Bromiwich et al., 215). A comparison of surface and 5 mb observations between the ERA-I and ASR version 1 during 27 found similar annual mean biases, with the ASR generally having smaller root-mean-square errors and higher correlations (Bromwich et al., 215). Comparing the ASR with the ERA-I in the Greenland region, Moore et al. (216) found that the ASR was more accurate for both high and low wind speeds, with a smaller root-mean-square error between the reanalysis winds and observations. We also use the ASR fields to compute time series of wind stress curl Wind-driven upwelling events We begin our study by objectively defining what we mean by a wind-driven upwelling event and comparing this to different wind metrics. We then construct a composite event using our 6 years of data, which enables us to robustly explore various aspects of upwelling on the Beaufort slope. Wind-driven upwelling is assumed to take place at the mooring site when the near-bottom potential density at mooring BS3 is greater than the climatological monthly mean value (where the climatology is computed over the 6-year record), and the mean alongcoast (15 T) wind is easterly (negative) over the upwelling period. On average, the hydrographic response of the water column

10 lags the alongcoast wind by t d = 21 h, which was taken into account when computing the event- mean alongcoast windspeed. A small fraction of upwelling events, with duration less than a day, had only a minor effect on the water column and hence are not considered in the study. Furthermore, we exclude events that lasted more than 17 days (12% of the events), because it was ambiguous if some of these were actually due to back-to-back storms (excluding these long events did not appreciably impact our results). Accordingly, a total of 115 upwelling events were identified over the 6-year record. The events occurred in all seasons, which is in line with previous studies (e.g. Schulze and Pickart, 212) Effect of wind To clarify the effect of the alongcoast wind on the upwelling events, we quantified the strength of each event by defining an upwelling index (UI) which is the time integral of the near- bottom potential density anomaly (i.e. subtracting out the climatological monthly mean) over each event. This definition accounts for both the magnitude and duration of the events. As an indicator of the alongcoast wind, the cumulative Ekman transport (Huyer et al., 1979) for each event was computed, * " #$ = +,* - & ' ( )( /(1 *.,* 2 3), (1) - where t s and t e are the start and end time of each upwelling event, respectively, & ' is the wind stress, f is the Coriolis parameter, and 1 2 = 125 kg m -3 is the reference density. Similar to the upwelling index, T CE takes into account the magnitude and length of the storm, and has been used in other studies as a measure of the wind-driven secondary circulation (e.g. Pisareva et al., 215). Comparing UI and T CE for all of the upwelling events over the six-year period, we find that the two quantities are significantly correlated (R=.71, with a confidence level of > 95%, Fig. 2a). This indicates that more offshore cumulative Ekman transport is generally associated with stronger

11 upwelling, which is not surprising. The best fit regression (black line), log UI =.62 log(" #$ ) 1.66, suggests a linear model for predicting UI from alongcoast wind. In addition to the Ekman transport driven by the easterly wind, we consider the Ekman pumping. In particular, we computed the mean local wind stress curl for each upwelling event using the ASR data (the data points near mooring BS3), and compared this to the corresponding value of UI (Fig. 2b). There is no obvious relationship between the upwelling index and the wind stress curl. In general, positive wind stress curl is upwelling-favorable and negative curl is downwelling-favorable. However, over the 6-year record, more than 5% (64 of 115) of the upwelling events occurred with negative wind stress curl. Furthermore, for the strongest upwelling events (UI > 18), the wind stress curl was close to zero. We also integrated the wind stress curl around the mooring BS3 during each upwelling event (not shown), which does not change the result. This implies that the upwelling in our study region is not related to the local wind stress curl, but instead is associated with coastal upwelling, which is consistent with the modeling results of Pickart et al. (211) General characteristics of upwelling To investigate the general features of upwelling, we constructed a composite event by averaging together the 115 individual events. The mean (median) duration of the events was 4.8 (2.9) days, and 68% of the events were shorter than 5 days. We defined a normalized time, t n, which ranges from at the beginning of the event to 1 at the end (recall that an event is defined as the time period over which the near-bottom potential density anomaly exceeds zero during easterly winds). We consider as well the conditions just prior to and after the upwelling (-.25 t n 1.25). Figure 3 displays the composites of alongcoast windspeed and near-bottom potential density anomaly. Since each of the events are independent, the uncertainties are reported as standard errors

12 (in this figure and subsequent figures). One sees that, on average, the wind intensifies prior to the onset of upwelling and peaks at 5.3 m s -1 shortly after the upwelling begins (roughly at t n =.1). This is consistent with the results of Schulze and Pickart (212) who found (using only two years of data) that upwelling often commenced when the easterly wind speed exceeded 4 m s -1. After peaking, the wind falls steadily during the remainder of the event to speeds less than 1 m s -1 at the end. The potential density anomaly for the composite event reaches values near.5 kg m -3, peaking near t n =.4. This is consistent with the above result that there is a delay between the wind forcing and the upwelling response. Note that, after the event, the density anomaly falls to the same level as prior to the upwelling. We also computed various hydrographic fields for the composite upwelling event (Fig. 4). Prior to the event, cold Pacific water is present in the lower part of the water column. This layer is lifted at the mooring site during the event and also warms, indicative of enhanced mixing. Interestingly there is an asymmetry in that a larger amount of winter water resides on the upper slope after the event ends. During the upwelling, the deep isohalines (and isopycnals) are quickly raised approximately 2 m by t n =.1. Notably, for the composite event, only a small amount of Atlantic water appears at the base of the mooring from t n =.25.6 (this is based on the Pacific- Atlantic water boundary from Nikolopoulos et al. (29)). The distribution of buoyancy frequency reveals that the upwelled dense Pacific water has enhanced stratification, and the middle portion of the water column, from 4 to 6 m, also becomes more strongly stratified (note that our hydrographic measurements only extend to 4 m) Velocity structure of upwelling Using the ADCP data at mooring BS3 we computed composites of the along-isobath and cross-isobath velocities (Fig. 5). The along-isobath direction was chosen to be 125 T, based on the

13 IBCAO v3 bathymetric product. Positive (negative) values refer to eastward (westward) in the along-isobath direction and offshore (onshore) in the cross-isobath direction, respectively. This is also the direction of the year-long mean eastward-flowing shelfbreak jet (Nikolopoulos et al., 29). One sees that the depth-averaged current reverses during the first part of the upwelling event when the easterly winds are strongest (compare Fig. 5a to Fig. 3a). The peak speed reaches.1 m s -1. For an average current width of 15 km (Brugler et al., 214) and depth of 15 m, this results in a temporary westward transport of.22 Sv, which is roughly two times larger than the undisturbed eastward transport of the jet (Nikolopoulos et al., 29). The depth-dependent along-isobath velocity (Fig. 5b) reveals that, prior to the onset of upwelling, the eastward-flowing shelfbreak jet is bottom-intensified (~.1 m s -1 ), while the flow in the upper layer has already been reversed. This is due to the intensifying easterly winds. Such a short response time of the shelfbreak jet to the winds is consistent with the statistical analysis of Pickart et al. (29) who computed a time lag of 8 hours, which is on the order of the inertial timescale. By the time the upwelling commences, the shelfbreak jet has been completely reversed and the westward flow reaches its peak speed (exceeding.15 m s -1 in the surface layer) around t n =.1. This is near the peak of the wind, indicating that the velocity responds very quickly to the easterly winds. About half way through the event the shelfbreak jet starts to re-establish at depth, and, at the end of the event, it is much stronger (.2 m s -1 ) than it was before the storm. This is in line with the results of Pickart et al. (211) who analyzed a single upwelling event in November 22. They demonstrated that an eastward-flowing rebound jet spun up at the end of the event which transported Atlantic water eastward. This was due to the discrepancy in timescales of the sea surface height response to changes in the wind versus the baroclinic water column response.

14 Our composite event demonstrates that the development of the post-storm rebound jet is a ubiquitous feature of the Beaufort slope. The depth-averaged cross-isobath velocity (Fig. 5c) is onshore over most of the event, with a peak speed of.2 m s -1. The depth-dependent structure (Fig. 5d) is that of onshore flow in the upper 6 m, offshore flow beneath this (to ~11 m), and onshore flow in the bottom layer. (Recall that the ADCP is situated roughly 2 m above the bottom, so our measurements do not capture the near-bottom flow.) The onshore flow at depth is of course required for the upwelling. This threelayer structure is different than the Ekman-like secondary circulation pattern described by Schulze and Pickart (212) and Pickart et al. (213b) during upwelling at this location. There are two important differences, however, between those analyses and ours. The earlier studies defined an alongstream coordinate system aligned with the mean transport vector (i.e. the flow averaged both vertically and laterally across the current), which changes orientation from storm to storm (Schulze and Pickart, 212) and even within a single storm (Pickart et al., 213b). Here we are interested in the upwelling that (by definition) takes place across the isobaths, hence we align our coordinate system with the bathymetry. The previous studies also considered the region of the outer shelf which is shoreward of the core of the shelfbreak jet. In such a coordinate frame the secondary flow on the outer shelf is accurately predicted by Ekman theory (Pickart et al., 213b). However, it is evident by comparing Figs. 5b and 5d that, for the composite event presented here, the reversed shelfbreak jet is oriented slightly onshore. That is, some of the westward alongstream flow is folded into the cross-isobath component. This likely overcomes the Ekman flow directed seaward across the isobaths in the surface layer. To elucidate this further, we diagnosed the vertically integrated along-isobath momentum balance. Due to the fact that we have only a single mooring, we are unable to evaluate the

15 divergence of the along-isobath momentum flux (along-isobath DMF) or cross-isobath momentum flux (cross-isobath DMF). To assess the magnitude of the former, we estimated the convergence of the isobaths along the upper slope in the vicinity of the mooring using the IBCAO v3 bathymetric data set. Assuming that the transport is conserved along the slope, this results in a small along-isobath change in the current speed, implying that this term is on average only ~15% of the wind stress term and thus can be ignored. With regard to the cross-isobath DMF, using data from the full SBI mooring array Pickart et al. (213b) calculated this quantity for a single upwelling event. They found that this force acted in the same manner as the surface stress term in reversing the shelfbreak jet, but was roughly 3/4 of the magnitude. As such, we assume that it has the same functional form as the wind stress calculated below but is smaller by 25%. (We note that Pickart et al. (213b) evaluated the force balance a bit shoreward of the core of the shelfbreak jet.) The terms in the momentum balance can be written as 334 GHHIJIKL*MNO C C( 2 D )F,E + C CQ STU 2 DR )F,E 3 #NKMNJM' 2 R )F,E XYU CV )F 1 2 CW,E [MO\ '*KI'' & 'Z ]N**N^ '*KI'' & ]Z =, (2) where x and y are the along-isobath and cross-isobath directions, respectively; u and v are the corresponding depth-dependent velocities; f is the Coriolis parameter; and 1 2 = 125 kg m -3 is the reference density. The terms on the left-hand side of (2) are the local acceleration of the verticallyintegrated along-isobath velocity, the cross-isobath DMF, the Coriolis force, the along-isobath pressure gradient force (PGF), the wind stress (& 'Z ), and the bottom stress (& ]Z ). The along-isobath PGF is determined as the residual (see also Pickart et al., 213b). We assume a linear drag law to estimate the bottom stress (Pringle, 22): & ]Z = 1 2 _`/a, (3) where H is the water depth, and r m s -1.

16 The evolution of the terms in (2) during the normalized upwelling event are presented in Fig. 6. Note that all of them are close to zero before and after the event (at t n =-.25 and t n =1.25). The acceleration term reflects the spinning up of the reversed jet followed by the re-establishment of the eastward-flowing shelfbreak jet, and, finally, the acceleration of the rebound jet. Before the upwelling begins, the wind stress and bottom stress are the same sign, but once the jet reverses through the water column they tend to balance each other. As such, the Coriolis force is largely responsible for the continued reversal of the along-isobath flow. Through roughly the first three quarters of the upwelling period the PGF tends to offset the Coriolis force. This is a reflection of the fact that the reversed jet is in geostrophic balance; in particular, the sea surface height is higher offshore of the jet (e.g. Pickart et al., 213b). As noted above, the orientation of the reversed jet is such that a component of the alongstream flow is directed across the isobaths over this time period. Hence a signature of the cross-stream geostrophic balance is manifested in the along-isobath force balance. During the last part of the event the Coriolis force in Fig. 6 goes to zero and the PGF largely balances the local acceleration which becomes larger at this time. This corresponds to the spin up of the rebound jet (Fig. 5b), and our interpretation is that the PGF is now mostly due to a west-to-east drop in sea surface height (consistent with the conclusions of Pickart et al., 213b). Overall then, we argue that the crossisobath flow during the composite upwelling event is not Ekman-like because of the orientation of the reversed jet, and that the large-scale zonal gradient in sea level drives the rebound jet at the end of the event. Both of these notions are consistent with the vertically integrated momentum balance

17 Upwelling of Atlantic water versus Pacific water As discussed in the introduction, during some upwelling events Atlantic water (AW) is advected onto the shelf, while the remaining events are characterized by only Pacific water (PW) in the near-bottom layer. This was noted by Schulze and Pickart (212) using the BS2 mooring data on the outer shelf from However, they did not consider the reasons behind the different cases. In this study we have 6 years of data available at mooring BS3, just seaward of the shelfbreak. We now investigate what dictates the two types of upwelling, which we refer to as AW-type and PW-type, respectively. Over the 6-year time period, 85 of the 115 upwelling events were AW-type, and the remaining 3 events were PW-type. This means that upwelling of AW occurs about three times more frequently than just PW alone Atmospheric forcing First we investigate the role that the atmospheric forcing has on the type of upwelling. Since the AW resides below the PW in the Canada Basin, one might expect simply that the stronger the winds, the greater the chance of drawing AW to the shelfbreak. This is not true, however. Previous studies have demonstrated that the winds in the western Beaufort Sea are largely dictated by two atmospheric centers of action, namely the Beaufort High and the Aleutian Low (Pickart et al., 213a; Brugler et al., 214; Lin et al., 216). Using the ASR reanalysis fields, we created composites of the sea level pressure (SLP) and 1-m winds for the AW-type upwelling events and the PW-type events (Fig. 7). For the former (Fig. 7a), a pronounced Beaufort High dominates the Canada Basin, while a deep Aleutian Low is centered over the Alaskan Peninsula (extending over a large portion of the Gulf of Alaska and Bering Sea). The SLP gradient between the two centers of action leads to the easterly winds in our study area. By contrast, PW-type upwelling occurs when the Beaufort High is more confined to the Beaufort Sea with no signature

18 of the Aleutian Low (Fig. 7b). Notably, the strength of the easterly wind at the mooring site is comparable in each case. The time series of alongcoast wind for the composite AW- and PW-type upwelling events are similar as well (not shown). Recall the relationship between the upwelling index (UI) and the cumulative Ekman transport (T CE ) presented in Section 3.1. In Fig. 2a we have distinguished between the AW- and PW-type events. Consistent with the above result that wind strength does not play a role, both types of events are present over the full range of T CE. However, as is clear from the statistically significant linear fits, PW-type upwelling requires stronger and/or longer wind forcing to attain the same upwelling strength (i.e. near-bottom potential density anomaly) compared to AW-type upwelling. Also, the largest values of UI are all AW events, which is not surprising since AW is denser than PW. These results raise the following question: how does the same strength wind lead to upwelling of Atlantic water in some instances and Pacific water in others? This is addressed below in Section Physical attributes To shed light on the character of the AW- versus PW-type upwelling, we constructed the composite event for each type using the same methodology applied above. Since the majority of the events are AW-type, this composite is qualitatively similar to the full composite (compare Fig. 8 to Figs. 4 and 5). One sees in Fig. 8 that Atlantic water is present over much of the event, and the PW-AW interface rises to about 12 m depth. The jet is reversed throughout the water column and is stronger than in the full composite (peaking at.2 m s -1 ). The rebound jet is also stronger. Not surprisingly, the cross-isobath flow in the surface layer is greater as well (in line with the above discussion).

19 The analogous composite fields for the PW-type upwelling show some significant differences (Fig. 9). Much less saline water reaches the shelfbreak (< 33 versus > 34). Another notable difference is that during PW events the hydrographic response is confined to deeper levels: shallower than 8 m the isopycnals remain flat during the event (Fig. 9a). The stratification is also much weaker than during the AW events. Despite the fact that, on average, the wind forcing is similar during both types of events, the shelfbreak jet only reverses in the top 6 m during PW events with a peak westward speed of just.7 m s -1 compared to.2 m s -1 for the AW events. The eastward-flowing rebound jet is also weaker. Lastly, the onshore flow in the upper layer is reduced during PW events, and, correspondingly, the offshore flow in the middle of the water column is stronger. The detailed statistics of the two types of events are compared in Table Seasonal influences on upwelling type The atmospheric patterns associated with the AW- and PW-type upwelling scenarios (Fig. 7) are similar to the seasonal atmospheric composites of Brugler et al. (214). Those authors divided the year into a cold season (September to February) and warm season (March to August). The former is reminiscent of the AW case, and the latter similar to the PW case (see Fig. 12 of Brugler et al. (214)). This implies that seasonality but not seasonal wind strength is a main factor driving the type of upwelling that occurs on the Beaufort slope. Using the 6-year record, we summed up the number of AW- versus PW-type upwelling events for each month (Fig. 1), which reveals a seasonal pattern. Nearly all of the PW events occur in the warm months, particularly in summer (June August). By contrast, there are relatively few AW events during that time period. Instead, the occurrence of AW events has two peaks: one

20 in spring (March May) and the other in fall (October December). What dictates this seasonal variation in upwelling type? 5.1. The Pacific water Atlantic water interface Using the first year of SBI mooring array data (August 22 August 23), Nikolopoulos et al. (29) constructed a time series of the PW-AW interface depth and noted that it deepens in summer and shoals in late-fall (no other mention was made of it). In light of the seasonality in upwelling noted above, this motivates us to consider variations in the PW-AW interface depth more carefully. The idea is that the type of upwelling that occurs is dependent on how readily each of the water types is accessible seaward of the boundary current. As such, we need to investigate the signals in the PW-AW interface away from the region where the upwelling takes place. The above results (Fig. 4) and previous studies indicate that the signature of upwelling is strong on the upper continental slope in particular at the BS3 mooring site. Therefore, to look for seasonality in the PW-AW interface depth, we need to consider moorings that are farther into the basin, i.e. not directly influenced by the upwelling activity. We computed the monthlyaveraged interface depth at the offshore SBI moorings (BS4 BS7) for the time period September 22 August 24 (Fig. 11a). All four time series vary in phase with each other and display a clear seasonal pattern, in line with that noted by Nikolopoulos et al. (29). To shed further light on this we also composited the PW-AW interface depth at each mooring site for the normalized upwelling event (Fig. 11b, where it should be noted that there are just 48 events in the composite because the full mooring array was deployed for only two years). This demonstrates that the upwelling signal was evident at moorings BS4 and BS5, but that there was very little upwelling signal at the two offshore-most sites BS6 and BS7 (there was a slight deflection of the interface of approximately 5 m).

21 Therefore, we averaged the interface depth at BS6 and BS7 and took this as a measure of the conditions offshore of the boundary current (BS6 failed in the second year, so only BS7 was used then). Notably, when the interface depth was shallower than approximately 16 m, all of the upwelling events during the two-year period were AW-type. Conversely, all of the PW-type upwelling events occurred when the boundary was deeper than this (some AW events occurred under this condition as well). This suggests that the interior interface depth plays a primary role in determining whether AW versus PW is upwelled to the shelfbreak Effect of wind stress curl What causes the seasonal change in the depth of the PW-AW interface? Atmospheric forcing is an obvious candidate. Many observational and modeling studies have shown that the anticyclonic wind stress curl in the Canada Basin drives Ekman transport that advects freshwater from the boundary to the Beaufort Gyre (e.g. Proshutinsky et al., 22; 29). The convergence of this transport in the interior then deepens the halocline. Seasonally, the Ekman pumping in the basin is weaker in the summer and stronger in the remainder of the year (Yang, 26), which in turn implies that the PW-AW interface is deeper in the winter. This is the opposite of what is shown in Fig. 11a, which suggests that the behavior of the interface just seaward of the boundary current (at the base of the continental slope) is different than in the center of the Beaufort Gyre. To investigate this, we correlated the monthly mean interface depth calculated above at the two offshore mooring sites with the monthly wind stress curl field over the Beaufort Sea and the Canada Basin using the ASR product. The wind stress curl was averaged into 1 1 bins to reduce noise, and the correlation coefficient determined between the PW-AW interface time series and the curl at each grid point. The spatial distribution of correlation (Fig. 12a) reveals a region of significant negative correlation in the shelfbreak/slope region of western Alaskan Beaufort Sea,

22 where the mooring array was located, and a positive correlation farther to the northwest in the Canada Basin. This indicates that the situation is different in the interior basin versus the nearboundary region. Using the 13-year ASR record we then computed the correlation between the wind stress curl close to mooring BS7 and the wind stress curl at each data point in the domain (Fig. 12b). This shows a band of high-correlation along the slope of the western Alaskan Beaufort Sea in the vicinity of the mooring array. As such, we chose the area delimited by the box in Fig. 12 as the relevant region for averaging the wind stress curl. The resulting monthly area-averaged wind stress curl for September 22 August 24 shows good agreement (R= -.63) with the monthly interface depth time series (Fig. 13): the interface becomes shallower when the wind stress curl increases, and vice versa. Note that the curl is negative during the summer months, so that there is Ekman pumping during the time period when the PW-AW interface is deepest and the PW upwelling events occur. This indicates that, while the wind stress curl does not drive the upwelling locally at the shelfbreak (Fig. 2b), offshore of the boundary the seasonal variation in curl impacts the accessibility of the AW to be drawn to the shelfbreak via the upwelling. We note that the changes in interface depth predicted by the wind stress curl (h = ( &/1 2 3) only accounts for 36% of the observed monthly changes on average. This is discussed in Section Atmospheric patterns We now return to the atmospheric fields to further clarify the seasonal variation of the wind stress curl near the boundary. Using the ASR product, we constructed a latitude-time distribution of climatological monthly mean wind stress curl over the period in the vicinity of the mooring array (Fig. 14). In the northern part of the domain the wind stress curl is negative associated with the Beaufort High (see also Fig. 7). However, in the southern part of the domain,

23 including over the land (south of the dashed grey line), the curl is positive. One sees that the zerocurl line is located south of the mooring array during the summer months and north of it remainder of the year. This seasonal migration explains the time series of wind stress curl in Fig. 13. What causes the latitudinal variation in the zero wind stress curl line? To answer this we composited the SLP for the periods during when the zero-curl line was north of its mean latitude plus one standard deviation (25 months), and south of its mean latitude plus one standard deviation (22 months). The former shows the Aleutian Low situated in the Gulf of Alaska and the Beaufort High displaced to the west (Fig. 15a). This is similar to the winter SLP pattern shown in Brugler et al. (214) (17 of 25 months in the composite of Fig. 15a are in winter). By contrast, the latter composite shows a well-developed Beaufort High with barely any signature of the Aleutian Low (Fig. 15b), in agreement with the summertime SLP pattern from Brugler et al. (214) (2 of 22 months comprising Fig. 15b are in summer). Notably, the SLP composite when the zero-curl line is located to the north is very close to that associated with the AW-type upwelling (compare Figs. 7a and 15a), while the SLP composite when the zero-curl line is to the south reflects the conditions when PW-type upwelling prevails (compare Figs. 7b and 15b). Hence we conclude that the north-south migration of the zero-curl line associated with the behavior of the two atmospheric centers of action dictates, to first order, the type of water that can be upwelled to the Beaufort shelfbreak Discussion Our calculations indicate that, while the wind stress curl is highly correlated with PW-AW interface, the degree of deflection of the interface implied by the Ekman pumping/suction accounts for only a portion of the observed interface changes. It is important to keep in mind, however, that

24 the spatial resolution of the ASR fields is 3 km, and if there were a higher resolution product then the curl signal would likely be significantly stronger. (Indeed, the ASR-derived curl is much larger than the curl computed using 8 km ERA-I reanalysis product.) Nonetheless, in light of the high positive correlation between the PW-AW interface at BS6 7 and the curl in the Canada Basin (Fig. 12a), there could also be a remote effect. In particular, the seasonal adiabatic migration of the pycnocline in Canada Basin could be compensated by an oppositely phased migration near the boundary in order to conserve volume (Proshutinsky, 1988). By assuming the Arctic Ocean has two layers in the vertical, Proshutinsky and Johnson (1997) found that, in response to wind forcing, the interface is raised in the center of the Arctic while it is depressed along the coasts, and vice versa. We also note that ice is not considered in this study, which may lead to an underestimate of the interface movement. As discussed in the introduction, mobile pack ice is able to enhance the surface stress on the sea surface (Häkkinen, 1986; Schulze and Pickart, 212), which in turn could lead to stronger values of the wind stress curl and larger isopycnal displacements. Another possibility for varying the interface height seaward of the boundary is steric changes to the water column. Such steric-driven pycnocline depth fluctuations are reflected in the varying sea level which has been well studied (e.g. McGregor et al., 212; Palanisamy et al., 215). Both currents and atmospheric systems make a signification contribution to the heat and freshwater fluxes within our study region. The shelfbreak jet transports a significant amount of freshwater and heat to the vicinity of the mooring, particularly in summer (Brugler et al., 214). As demonstrated by von Appen and Pickart (212), the summertime structure of the jet is baroclinically unstable and hence is likely to shed eddies of Pacific water offshore (e.g. Spall et al., 28). With regard to atmospheric forcing, even a single storm can flux a significant amount of heat and freshwater from the shelf to the interior (Pickart et al., 213b). Consequently, we can

25 expect an enhanced presence of relatively warm and fresh Pacific summer water in the Alaskan Beaufort Sea during the summer months which would in turn depress the pycnocline downward. Previous studies, as well as the results presented here, have demonstrated that the Beaufort shelfbreak jet is highly sensitive to the wind (e.g. Nikolopoulos et al., 29; Pickart et al., 29; Brugler et al., 214). To further assess the effect of wind on the jet, we considered a wind strength index, f ' = D gmo\ /D ]hni, where D gmo\ and D ]hni are the wind-driven and buoyancy-driven velocity scales, respectively (Whitney and Garvine, 25; Sutherland and Pickart, 28). In 556 particular, D gmo\ = j klm # no j # `gmo\ and D ]hni = qr E p s, where 1 LMK (1) is the air (water) density, t u2 (t S ) is the surface (bottom) drag coefficient, `gmo\ is the wind speed, v w h is the internal wave speed, x is the dimensionless current width. W s > 1 indicates that the wind plays an essential role in the existence of the current (e.g. W s = 4. in Scottish Coastal Current), while W s < 1 suggests that buoyancy forcing is dominant (e.g. W s =.1 for the Alaskan Coastal Current) (Whitney and Garvine, 25). Using all six years of data, we calculate a W s of ~.3, implying that the Beaufort shelfbreak jet is not predominantly a wind-driven feature. This is in line with the results of Lin et al. (216) who determined that the wind-driven component of the shelfbreak jet does not account for the dominant seasonal variation in total transport. During upwelling events, however, the shelfbreak jet is readily reversed by easterly winds and under these conditions W s > 1. This is similar to the coastal current in the South Atlantic Bight for which W s =.6 on average, but during upwelling events it increases to 1.7 (Whitney and Garvine, 25). One should keep in mind that, in summer, the Beaufort shelfbreak jet is the extension of the Alaskan Coastal Current which is predominantly buoyancy-driven (Royer, 1983). Overall, the value of W s computed here suggests that, while not primarily wind-driven, the Beaufort shelfbreak jet is easily modified by the wind.